1. Field of the Invention
The present invention relates to image processing apparatuses and image processing methods, and more particularly, to a technology for protecting the copyright of digital image information.
2. Description of the Related Art
Image input apparatuses, such as digital cameras, for recording images in a recording medium as digital data, rather than in photographic paper or 8-mm film, have recently become commercially available.
In general, digital data can be easily processed, unlike analog data, and therefore can be readily modified, falsified, forged, or synthesized. Consequently, digital data has lower authentication than conventional film (silver halide) photographs and is difficult to be used as a proof.
To solve the foregoing problem, there has been proposed a technology for detecting modification, falsification, forgery, and synthesis of digital data. For example, a system employing a hash function and a public-key encryption method has been proposed.
A conventional system will be described below by referring to FIG. 1. The public-key encryption method uses a private key and a public key different from each other, makes the public key public, and maintains the private key confidential.
The structure and operation of a transmission side (output side) will be described first.
(1) Digital data M is compressed with the use of a hash function H to obtain a constant-length output h.
(2) Data h is processed with the use of a private key Ke to obtain an output s. This output s is called the digital signature data.
(3) An output circuit outputs the digital signature data s and the digital data M as a set.
The structure and operation of a receiving side (detection side) will be described next.
(4) The digital data M and the corresponding digital signature data s are input.
(5) The digital signature data s is processed with the use of the public key Kd corresponding to the private key Ke to obtain an output h″.
(6) The digital data M is compressed with the use of the same hash function H as that used in the transmission side to obtain an output h′.
(7) A comparison circuit compares the output h″ obtained in (5) with the output h′ obtained in (6). When they match, it is determined that the input digital data M is correct data to which illegitimate processing is not applied. If they do not match, it is determined that illegitimate processing has been applied to the input digital data M.
As described above, in the conventional system, modification, falsification, forgery, and synthesis of the digital data M are detected by the use of the digital signature data s, which is generated by the hash function H and the private key Ke.
This system, however, has the following drawbacks.
An encryption circuit and a decryption circuit used in the public-key cryptosystem have complicated circuit structures and are difficult to be made compact. In addition, a vast amount of calculations is required in the circuits and the processing time is long. Since the public-key encryption method needs calculations of powers and remainders, its calculations are more complicated and enormous than those required for the secret-key cryptosystem (an encryption method in which an encryption key is the same as a decryption key) and therefore it is very difficult to increase the processing speed. In other words, in the conventional system, it is difficult to achieve fast processing and to make the system compact.
To increase the processing speed, it is necessary to use a higher-performance central processing unit (CPU) and a larger-capacity memory to improve the performance of hardware. With such a structure, however, the entire system becomes large and its cost increases. A compact, inexpensive, high-speed system cannot be provided for the user.